The present invention relates to microwave hybrid integrated circuits in general, and more particularly, to an integral ground plane soft-dielectric substrate microstrip circuit locally modified to embed at least one suspended substrate stripline region for high Q circuit elements and a method for making the same.
From a manufacturing cost view point, it is well recognized that microstrip technology appears to be the optimum in the implementation of a large class of microwave circuits. The primary advantage in the manufacturing process of microstrip is the use of a one step photolithographic technique for preparing continuously the various microstrip circuits over a large surface area. However, since the microstrip circuit is an open circuit, that is elements like transistors, chip capacitors, chip resistors, . . . , etc., for example, can be assembled on the top of the printed circuit region, it is an unbalanced microwave circuit and therefore, exhibits low Q levels, on the order of 100-200, for example.
Another form of microwave circuit implementation, generally referred to as stripline, includes a stripline printed circuit sandwiched between two solid thick dielectric layers which inturn are sandwiched between two ground planes resulting in a three conductor system. An example of this strip line form can be found under the trade name "Triplate" manufactured by Sanders Associates. While achieving a higher Q level, say on the order of 400, for example, the stripline structure is limited in going any higher because of the solid dielectric filling between the printed circuit conductor and ground planes.
One way of increasing the Q levels of the stripline implementation of microwave circuit is to eliminate or reduce the solid dielectric filling which leads to a microwave circuit structure having a thin layer of dielectric supported in air between two supporting structures and a stripline microwave circuit formed on the dielectric layer. In this structure, two ground planes are supported on top and below the microwave conductor pattern formed on the dielectric. The air spacing between the ground planes and the printed circuit pattern above and below is much greater than the dielectric thickness. This configuration is commonly referred to as suspended substrate stripline and may achieve Q levels on the order of 500-1000, for example.
The problem which presents itself in many microwave circuits is the need for only a few relatively small regions of the overall microwave circuit that may include only one or two circuit elements with very high Q circuit requirements. For example, in a low noise microwave oscillator/buffer amplifier circuit, only one resonator in the oscillator circuit requires high Q levels to maintain low noise output. When the dielectric constant and thickness of the dielectric layer are selected solely to satisfy this criteria for the critical application, the remainder of the microwave circuitry suffers somewhat from this undesirable compromise. For example, the use of dielectric thickness in excess of 0.025 inches to improve the Q levels often introduces problems with reliability in groun connections due to thermal expansion stresses. Apparently, it is not only the technology to achieve greater Q levels in the microwave circuits which is needed, but also a way of interfacing the various technologies to achieve the performance of high Q elements in one medium and lower Q elements in another while maintaining the overall advantages of both.
Generally, when it is determined economically attractive, the entire microwave circuitry may be implemented with microstrip technology except for those regions which required high Q levels. For these regions, the microstrip circuitry may be terminated and the remaining high Q circuitry built in a different medium like coaxial section, for example. The two regions may then be physically connected together with connectors, screws or other mechanical devices for structural support. This interconnection of microstrip with coaxial or microstrip with other types of high Q mediums generally results in a real discontinuity in current flow. Not only do these mechanical type interconnections render a high manufacturing cost, but they also include a number of structural and fabricational disadvantages.
Therefore, for those microstrip microwave circuits which include resonators and filters requiring another medium to support a higher unloaded Q than can normally be provided in the microstrip medium, it is of paramount importance to maintain a structural integrity for RF current flow continuity between the high and low Q mediums while achieving an integralness in the fabricational process to reduce the high cost of manufacturing and alleviate the mechanical problems associated with interfacing the different microwave circuit mediums.